The present invention relates to a laser scanning device for emitting an amplitude-modulated scanning laser beam, and to a method of modulating the laser beam employed in such a laser scanning device.
Conventionally, a laser scanning device utilizing a laser diode which is controlled to emit an amplitude-modulated laser beam is known. Such a laser scanning device is used, for example, to form a latent image on a photoconductive material of an imaging device such as a laser beam printer. Recently, printers capable of printing not only a black and white image but also an image having gradation are desired by users.
Generally, in order to form the image having gradation, the intensity of the laser beam is varied in accordance with the gradation level of the image data. If the laser scanning device as described above is used, in an imaging device forming the gradation image, in order to make the intensity of the emitted laser beam for each gradation step stable, an APC (automatic power controlling) is performed. When the APC is performed, a white level intensity and/or a black level intensity of the emitted laser beam are adjusted during a period where the scanning beam does not affect the imaging process, more specifically, the laser beam emitted by the laser diode is received by a photo diode. Across the photo diode, electrical current corresponding to the received amount of light (i.e., laser beam) is generated. Based on the current generated by the photo diode, the intensity of the beam emitted by the laser diode is determined and then the laser diode is feed-back controlled so that the black level intensity and white level intensity of the laser beam have predetermined, or specified values, respectively. When the intensity of each of the black and white level is determined, the current to flow through the laser diode to obtain the white level and black level laser beam can be determined. After the current values to emit the black and white levels of the laser beams are determined, the difference between the black and white levels is divided in accordance with the number of gradation steps, and the laser diode is controlled to emit the laser beam in accordance with the image data having the gradation information.
The photo diode has, however its intrinsic errors in terms of its sensitivity. That is, even if the same amount of light is incident to the photo diode, generated current with respect to received amount of light may depend on respective photo diodes. Further, depending on the ambient temperature, the characteristics of the laser diode varies. Therefore, as the laser scanning device (i.e., the laser diode) is used, the temperature may increase and the characteristics may change. If the intensity of the laser beam is not proportional to the current flowing through the laser diode, the gradation level may not be accurate even though the intensities of the black and white level are adjusted during the APC operation.